The present invention relates to the field of electronics, and in particular, to a voltage follower and a corresponding method of regulating the same.
Often in the implementation of complex electronic systems, it is not possible to connect the output of a certain stage to the input of a stage downstream in a signal path. In particular, this is not possible when the downstream stage is to be fed with a small output impedance or with a power greater than that provided by the preceding stage. Usually, in these situations, the upstream stage is coupled to the downstream stage through a voltage follower.
A voltage follower is a voltage amplifier that outputs a voltage equal to its input voltage independent of the current being absorbed by its electrical load. A typical voltage follower is schematically depicted in FIG. 1, and includes a follower stage formed by the transistors T1 and T2, which are biased by a constant current generator. The output voltage VOUT of the follower stage is a replica of the input voltage Vrif and does not depend on the output current IOUT circulating in the transistor T2. The voltage follower is generally formed using BJT technology because of its simplicity, and because of the good tracking between input and output voltages that can be ensured.
The current generator generates a bias current I such that the transistor T2 may output the desired maximum current IOUT,MAX to the load. Since xcex2min is the minimum gain of the transistor T2, the following relationship must hold:
I=IOUT,MAX/xcex2minxe2x80x83xe2x80x83(1)
The current circulating in the transistor T1 ranges from a minimum value when the transistor T2 outputs the maximum current IOUT,MAX, up to a maximum value I when the output current IOUT is null.
A current almost equal to I circulates in the transistor T1 as long as a relatively small current IOUT circulates in the transistor T2. Therefore, the transistor T1 must be properly dimensioned to have a substantially constant base-emitter gain even when the current circulating in it becomes relatively large.
The circuit of FIG. 1 is not very efficient when a relatively small current is being supplied to the load. The power provided by the supply Ps is:
Ps=Vccxc2x7I+Vccxc2x7IOUT=Vccxc2x7[(IOUT,MAX/xcex2min)+IOUT]xe2x80x83xe2x80x83(2)
and the power delivered to the load is:
PLOAD=VOUTxc2x7IOUTxe2x80x83xe2x80x83(3)
Therefore, the power dissipation is:
xe2x80x83PDISS=Psxe2x88x92PLOAD=Vccxc2x7[(IOUT,MAX/xcex2min)+IOUT]xe2x88x92VOUTxc2x7IOUTxe2x80x83xe2x80x83(4)
The power dissipation of the voltage follower of FIG. 1 becomes relatively large when IOUT is relatively small, as it may be easily inferred from equation (4). Thus, there is a need for a voltage follower capable of delivering to a load a current IOUT that may assume a certain maximum value IOUT,MAX, while dissipating a significantly reduced power when IOUT becomes relatively small.
In view of the foregoing background, an object of the present invention is to provide a voltage follower that is efficient for driving a load that generally absorbs a relatively small current and only occasionally a relatively large current.
The voltage follower according to the present invention dissipates less power than the voltage followers of the prior art as long as it delivers a relatively small output current. This is because the follower stage of the voltage follower is biased by a current that is progressively reduced as the output current delivered to the load decreases. According to one embodiment, the voltage follower comprises a follower stage formed by a pair of bipolar junction transistors, electrically in cascade, and commonly biased by a current generator.
The voltage follower dissipates a relatively reduced power when the output current is small because it comprises a second current generator. The second current generator is connected between the current node of the input transistor of the voltage follower and a common potential node of the stage. The first bias current generator is in the form of a current mirror. A diode connected transistor of the current mirror is coupled through a cascode stage to the current node of the first transistor connected to the second current generator.
Another aspect of the present invention is directed to a method of regulating a voltage follower driving a load. The voltage follower comprises a follower stage composed of a pair of bipolar junction transistors in cascade, and which are biased by an adjustable current generator. The method comprises progressively increasing or decreasing the bias current provided by the adjustable current generator as a function of a feedback signal representing the current that the output transistor of the pair of transistors delivers to the load.